1. Field of the invention
The present invention relates to a semiconductor device and a method for the manufacture thereof.
2. Description of the Related Art
GaN-based semiconductor materials, represented by gallium nitride have attracted public attention as materials for light-emitting diodes (LEDs) or laser diodes (LDs), since they emit blue-violet light at high efficiency. Above all, LDs are expected as the light sources for large-capacity optical disk devices, and in recent years, high-output LDs have been energetically performed as the light sources for writing.
Heretofore, semiconductor devices using GaN-based semiconductor materials have been fabricated utilizing the substrates of heterogeneous materials, such as sapphire and SiC. Specifically, a wurtzite-type GaN (0001) layer has been grown on a sapphire substrate or a SiC substrate using a two-step growing method, and a device structure has been fabricated using such a GaN layer as the substrate. This is because high-quality bulk GaN single crystal substrates have not been available.
However, when these heterogeneous substrates were used, a high-density crystal transition was introduced in the GaN layer due to difference in the lattice constants between the substrate and GaN, and it was difficult to obtain high-quality single crystals. Furthermore, sapphire substrates have various problems in practice; such as poor heat dissipation of the device characteristic due to low thermal conductivity; difficulty of mirror in the fabrication of LDs due to difference in cleaved surfaces between GaN and sapphire; and impossibility of fabricating back-electrode-type devices due to an insulator.
Under such situations, a technique for obtaining high-quality GaN substrates of low crystal transition has been studied by combining a GaN thick film growing technique utilizing HVPE (Hybrid Vapor Phase Epitaxy) and a crystal transition reducing technique. By using a GaN substrate having good thermal conduction characteristics and electric conduction characteristics, the improvement of heat-dissipation properties and the realization of a back-electrode-type semiconductor laser are expected. It is considered that the devices on GaN substrate will be the mainstream in the future.
When a method for manufacturing a semiconductor device using a GaN substrate is adopted, a large number of problems in processing must be solved. As one of such problems, an important technical problem is how to divide a semiconductor layer grown on a substrate into dies. Heretofore, since a red light emitting semiconductor laser diodes used in communications or DVDs has used a semiconductor material having a cubic system structure, when the end surface of a laser resonator is perpendicular to the chip separation direction, the cleavage plane of the crystal can be utilized in both directions. However, a GaN substrate having a wurtzite-type crystal structure has no cleaved plane in two directions orthogonal to each other of the wafer surface, and a rectangular chip cannot be formed using cleavage only. Since GaN is hexagonal system, if the <1-100> direction is the cleaved surface (LD end face), the cleaving direction, which is the direction easiest to cleave, is present only in the 60° directions thereof. Therefore, in order to separate a laser device composed of laser end surfaces from a stripe direction, stripe intervals must be wide, lowering the yield of devices and increasing the costs.
As a method for chip separation of a nitride semiconductor laser, Japanese Patent Application Laid-Open No. 2002-185085 discloses a method for separating devices wherein the back face of a substrate is scribed in the (11-20) direction to make the roughness thereof 300 angstrom or below. Japanese Patent Application Laid-Open No. 2001-85736 discloses a technique wherein a notch is formed on the GaN substrate side of a wafer, the surface of the crystal-growing side is scribed, and a roller is pressed onto the crystal-growing side to fabricated a chip.
However, when these methods are adopted, an additional process for reducing the roughness of the substrate is required, causing the process efficiency to lower and the production costs to increase.
The GaN substrate is very hard, and if it is cut in the plane other than the cleavage plane, cracks are easily produced. Therefore, during chip separation, crystal defects have often occurred, or cracks have often been produced.
Also when the LD end surface is coated, the coating has often been peeled off because the end surface is scribed.
Such problems are especially marked in regions where dislocation density is low.
The present invention has been devised considering the above-described circumstance, and it is the object of the present invention to provide a semiconductor device that excels in manufacturing efficiency and device reliability, and a method for manufacturing such a semiconductor device.